If you change your ticket within 24 hours of booking, you will have to pay the difference. According to Westjet Cancellation Policy, On the other hand, if you cancel your reservation within 24 hours of booking, the ticket cost will be fully refunded.
However, for same-day flights, i.e. flights departing within 24 hours of booking, a cancellation fee will be charged. You will also get a refund from your travel bank.